I have noticed that the 12v battery normally tests at 12.2 v or approx. 65% when driving daily. I assume that this is by design. I know that it just serves low amperage loads and is not needed for cranking a high draw starter motor. I wonder if it is better for its life expectancy or for some other reason to maintain it at a reduced state of charge. I have always tried to keep all my automotive batteries in a state of full charge, but everything about this car is different, so... Why would they engineer this charging circuit to maintain the 12v battery at less than a full state of charge? Inquiring minds need to know.
They don't design for lower vōtage. As a battery ages it loses capacity. Which means normal parasitic draw or a load test will show a lower voltage. When you first turn off the car there are still many loads for a few minutes or longer. Overnight the normal parasitic draw (without active charging) reduces the voltage. Lower capacity reduces voltage more. Its the metric. The only exception is if your car has excessive parasitic draw. This can kill your 12v batteries ability to Ready to a period of days rather than weeks. 12.2v is not bad; its just reflecting the battery is not new. Since it does not have a high current starter to spin normal rules of thumb don't apply. I sometimes see 10.5v readying a hybrid.
In a previous post, I was thinking the same thing and replaced my 7 year old battery. The new battery which I fully charged before placing it into service, tests fine but seems to be maintained by the charging system at a lower than expected voltage. I just charged the battery to 100% and tested it with my Solar tester. The battery is rated at 550cca and tests at 711cca.
I assume you are using a real-time DC voltmeter to determine this? Beginning with the Gen 4 in 2016, the 12V DC charging system is "smart" and reduces the charging voltage to 12.2 volts as you noted when the battery is re-charged, but it should start charging at the usual 14.4 volts in the beginning of the cycle. As you continue to operate the vehicle, the charging voltage reduces, and will cycle up and down to keep the 12V battery at full charge.
I'm using a Fluke 87 multimeter. I'm pretty sure that the design is to keep this battery at a charge less than what I'm used to seeing. I have never had a problem and shouldn't even be concerned other than wanting to know how and why it works this way. I would be interested in other 4th gen owners readings. I might get one of the voltage meters that plugs into the 12v outlet, but if it is not accurate, it would just be misleading, so... FuelMiser - Thanks for the good information. At 12.2v, my battery tester tells me that the SOC is 65% and that the battery needs to be charged. Seems like incorrect information for this particular application.
I had no idea there were so many previous threads. PIPLosAngeles posted a couple years ago - " I've discovered that Toyota intentionally designed Gen IV Prii (Primes included) to reduce the output of the DC-DC converter when the car is in drive from 14+v to ~12.5v." I wonder if this extends the life of the battery.
Maybe it just lets more energy be used for moving the car, when you're moving. Bumps the charge current up when you're stopped, might as well use it then anyway. I notice the same behavior in my Gen 3. Maybe mine doesn't drop as far as 12.5 though. My Gen 3 seems not to drop the voltage if it thinks the 12 V battery is very low on charge. Then it will keep the charge voltage higher. Graphs of datalogged voltage seem to show occasional short dips down below charging voltage, probably for an ECU to gauge how well charged the battery is.
